This invention relates to a method and arrangement for limiting the starting current in a power supply.
The state immediately following the switching on of the input voltage in a switching power supply, wherein there occur current spikes greater than in the normal operating state, is often called an inrush state. The normal operating state refers here to the steady operating state following the inrush state in the switching power supply. As the operating voltage is switched on in a power supply, the uncharged capacitor at the input side of the switching power supply acts as a short circuit. A large current will then flow through the circuit, called the starting, or inrush, current. The current may even be so large that it damages components in the circuit. Therefore it is practical that the starting current be limited in some way so that the circuit will remain intact and function as intended.
Several methods have been developed in order to limit the starting current. One of the most popular methods is to connect thermistors in series with the input. A bypass circuit is provided for the thermistors using a FET or a relay, for example. Operation of an NTC thermistor is based on the fact that as the operating voltage is switched on the resistance of the thermistor is of the order of a few ohms, but as it warms up, the resistance drops to near zero whereby the current flows through the thermistor. The thermistor may be an NTC or a PTC type thermistor. Fixed resistors have also been used. The problem with this method is that a separate bypass circuit for said resistors will be complex.
A second way of limiting the starting current is to use a separate winding. This method finds particular utility in a so-called high side circuit. In a high side circuit, a separate winding is used to generate from the input voltage a higher voltage to switch on the starting current bypass circuit. The disadvantage of this arrangement is its price, for a separate winding is expensive and requires other additional components for the protection of the gate or base of the starting current transistor. If the circuit utilizes a p-channel MOSFET or a pnp transistor, the problem will be that the components consume power, degrading the efficiency of the power supply.
FIG. 1a shows a third way of realizing starting current limitation in a switching power supply. The switching power supply gets its operating voltage from source V1 which may be a battery or a similar voltage source. The starting current is limited by a limiting resistor R1. The starting current MOSFET Q2 is at first non-conductive so the current flows through resistor R1 which limits the flow of current. The input capacitor C1 begins to charge up. As the input capacitor C1 has reached a sufficient charge, which is detected in the START/PWM block, which outputs a control to the gate of MOSFET Q1, the switching power supply starts operating. When MOSFET Q1 is not conducting, the energy stored in the windings of the transformer is fed via diode D2 to an RC circuit comprised of a resistor R5 and capacitor C3. The current coming through diode D2 is at first stored in capacitor C3 from where it is discharged to the gate of MOSFET Q2. MOSFET Q2 begins to conduct. The voltage across the gate is determined by resistors R2 and R4 which in an embodiment according to the circuit are of the order of 100 kxcexa9. Because of the large resistances the circuit has the disadvantage of being slow, for the RC time constant determined by the resistors together with the gate capacitance of Q2 is large. The values of resistors R2 and R4 cannot be reduced in the solution according to FIG. 1a, for the voltage level across capacitor C3 may be up to 50 V, which is enough to break MOSFET Q2. A zener diode D1 is placed between the gate and source of transistor Q2 in order to prevent the gate voltage from rising.
On the secondary side the secondary voltage is rectified by a diode pair D3. Energy is stored in choke L2. The voltage is filtered by choke L2 and capacitor C5. The output voltage is fed into the load R13. Capacitor C4 and resistor R10 make an attenuator circuit. The operation of the secondary of the switching power supply depicted in FIG. 1a is not described in more detail as it is substantially irrelevant from the point of view of the invention.
FIG. 1b illustrates the generation of the gate voltage UG of the starting current MOSFET in the circuit described above. UPWM represents a pulse coming from the starting block/pulse width modulator START/PWM. As can be seen from FIG. 1b, the starting current MOSFET does not become completely conductive until about the tenth cycle, as the MOSFET gate threshold voltage UK is exceeded. The slowness of the circuit makes the operating voltage of the switcher drop, as capacitor C1 is charged only through resistor R1. In the worst case, capacitor C1 will not be sufficiently charged, in which case the switching power supply stops operating.
Another problem of the circuit is the so-called Miller capacitance appearing between the drain and source of a MOSFET. Since the impedance between the gate and source is fairly high, even a small current may cause the MOSFET erroneously to conduct through the Miller capacitance. Such a situation may arise e.g. when the input voltage is rapidly switched on in the circuit.
An object of the present invention is to provide a method and an arrangement for limiting the starting current in a power supply, which arrangement can be realized through a structural solution as simple as possible and using as few components as possible, and the operation of which is fast enough when the invention is applied in switching power supplies.
The objects of the invention are achieved by taking the control signals for the starting current MOSFET from a so-called clamp circuit, using capacitive voltage division.
The method according to the invention is characterized by what is specified in the characterizing part of the independent claim 1. The arrangement according to the invention is characterized by what is specified in the characterizing part of the independent claim 2. The switching power supply according to the invention is characterized by what is specified in the characterizing part of the independent claim 5. Other advantageous embodiments of the invention are presented in the dependent claims.
In accordance with the invention the starting current limiting circuit comprises as few components as possible and it is simple. Since the control signal for the starting current MOSFET is taken from a clamp circuit, using capacitive voltage division, the resulting control circuit is very fast. In addition, the circuit according to the invention has the advantage that the starting current MOSFET is not switched on when the operating voltages are switched on, because the impedance between the MOSFET gate and source is rather small due to the large capacitance and low-value resistor. Therefore, in a switching situation a small current pulse coming through the Miller capacitance cannot raise the voltage at the MOSFET gate enough so as to make the MOSFET conductive.
A further advantage of the invention, in addition to the small number of components and simple structure, is that many of the components used are of similar type. For example, the starting current MOSFET may be an n-channel MOSFET similar to that used as a switching transistor in the switcher proper. Utilization of similar components reduces manufacturing costs of the power supply as the purchase quantities of a given type of component are bigger.